1. Field of the Invention
The present invention relates to a fluid dynamic bearing device rotatably supporting a shaft member by a dynamic pressure action in a non-contact fashion through the intermediation of a lubricant film generated in a radial bearing clearance. This bearing device is suitable for use in a spindle motor of an information apparatus, for example, a magnetic disc apparatus, such as an HDD or an FDD, an optical disc apparatus, such as a CD-ROM, a CD-R/RW, or a DVD-ROM/RAM, or a magneto-optical disc apparatus, such as an MD or an MO, the polygon scanner motor of a laser beam printer (LBP), a color wheel for a projector, or the small motor of an electric apparatus, such as an axial flow fan.
2. Related Background Art
Apart from high rotational accuracy, an improvement in speed, a reduction in cost, a reduction in noise, etc. are required of the motors as mentioned above. One of the factors determining such requisite performances is the bearing supporting the spindle of the motor. Recently, use of a fluid dynamic bearing superior in the above requisite performances is being considered, or such a fluid dynamic bearing has been actually put into practical use. This type of fluid dynamic bearing is roughly classified into two categories: a dynamic pressure bearing equipped with a dynamic pressure generating means for generating dynamic pressure in a lubricant in a bearing clearance, and a circular bearing equipped with no such dynamic pressure generating means (a bearing whose bearing surface is of a circular configuration).
A fluid dynamic bearing device using such a fluid dynamic bearing comprises a radial bearing portion supporting a shaft member in a radial direction, and a thrust bearing portion supporting it in a thrust direction. Usually, in a fluid dynamic bearing device, a fluid dynamic bearing is used in the radial bearing portion, and a fluid dynamic bearing or a pivot bearing supporting the axial end of the shaft member in a contact fashion with a thrust plate is used in the thrust bearing portion.
In a fluid dynamic bearing device whose thrust bearing portion is formed by a pivot bearing, the shaft member is pressed against the thrust plate by a magnetic force between the rotor and stator of the motor, whereby the shaft member is regulated in its axial movement to the side opposite to the thrust plate. However, when an impact load or the like in excess of the above-mentioned magnetic force is applied to the motor, or when the motor is used in an upright or a horizontal position, there is the possibility of the shaft member moving axially to the side opposite to the thrust plate to be detached from the bearing sleeve.
In view of this, to prevent such detachment of the shaft member, there is conventionally provided a flange-like detachment preventing portion in the outer periphery of the shaft member, wherein the detachment preventing portion is axially engaged with an end surface of the radial bearing (JP 2003-294049 A). Alternatively, a detachment preventing member is provided on the stationary side, and is engaged with a recess in the shaft (JP 2000-171712 A).
Both of the conventional techniques as disclosed in the above-mentioned publications require an additional member for preventing detachment. Thus, an increase in the number of parts is involved. Further, it is necessary to provide a step for mounting this additional member, resulting in an increase in cost. At the same time, to install this detachment preventing member, it is necessary to additionally secure an axial mounting space in the bearing device, which impedes a reduction in the size of the fluid dynamic bearing device.